Hoplo's are fairly social,so the number depends on the size of the tank,but in a 75g I'd hold it down to 2-3 as they can get 10" over time.
A personal favorite is Dianema urostriatum (Flagtail Catfish). Smaller at 5" and also social...fine with milder mid-size cichlids...it will often shoal at the mid-level when kept in groups over 3. Natural decor would be a sand substrate with tangled driftwoods,plants not a necessity but you could put some Anubias (easy care) up on the driftwood.
Going with a sand substrate,and no rooted plants,give a Geophagus species like Tapajos Redhead a try. They have "O-like personality" too. 6" adult size.
75g could support a Blue Acara,a pair of Redheads,3 Flagtails,possibly a small school of mid-size Tetras...but might look "busy" later once some size is on everyone.